One of our local players, Collins, has written a great tactica on the use of Eagles that I thought I'd share with all of you.  It's a nice list of the tricks he employs with his (four!!) Eagles during games.  Enjoy!
Trick #1, 'The Classic' War Machine  Hunting
I thought that we may as well start off with a  classic trick that I am sure everyone is familiar with. And that is  warmachine hunting. Through my experience, most of all of my opponents  warmachine crew have been s3 and ws2. That means that when you charge  that warmachine with your eagle on turn two, which is almost inevitable,  your opponents will be hitting you on 5s, and wounding you on 5s.  Therefor, they only have 1/3 chance of doing a single wound to you. You,  however, will be hitting on 3s, and wounding on 3s as well, so you  should be getting 1.5555 wounds on your opponent (including your eagles  stomp attack). That means you will be winning combat mostly every time. 1  from the charge, and 1-2 for wounds. From my experience, warmachines  have low leadership, and will usually break even when beaten by 2-3  points.
A tip I would like to point out to everyone, is that your  opponent will like sticking more than one warmachine together, usually  on a hill. This means that if in your first turn, you move your eagle up  on his flank and within charging distance of one warmachine, you can  usually charge the closest one, beat it and run it down, while at the  same time overrunning into his second warmachine, successfully taking  out 100-200 points with your single 50 point eagle. Also, if you get  this kind of charge off in your second turn, then you only are giving  your opponents warmachines 1-2 turns to shoot, depending on who goes  first.
Trick #2, 'Wizard  Assassination'
Here is another classic that most of you  must already know as well. Don't worry old experienced players, we'll  get into some more complex tricks later, but this is for those of you  who are relatively new and still need to learn the old tricks. Anyways,  what you do here usually takes 2 or more eagles, but the pay off is well  worth it. What you do is you charge in an eagle to the front of an  opponents unit that is holding their oh-so-powerful-mage. Then, you  direct all of the eagles attacks at your opponents mage. Because mages  are generally fragile and do not have much protection, you should be  getting 1.55555 wounds (stomp included). You're eagle will then surely  be killed afterward. However, in your next turn you do the same thing,  charge in with a second eagle. With two onslaughts of this, you should  be averaging 3 wounds on their mage, and this is usually enough to kill  it. And trust me, 100 points is well worth the points for your opponents  mage, and stopping their magic relatively early in the game.
Trick #3, 'The Run-Down'
Now  here is where things get interesting. This is the one trick that my  opponents question/hate the most (other than Trick #5..). Let me set the  scene for you:
---------------------------------------------------------------------
---------------------------------------------------------------------
---------------------------------------------------------------------
---------------------------------------------------------------------
----[EEEEEEEEEEEEEEEEEEEEEEE]------------------------------------------
----[EEEEEEEEEEEEEEEEEEEEEEE]------------------------------------------
----[EEEEEEEEEEEEEEEEEEEEEEE]------[EEEEEEEEEEEEEE]----------------------
----[EEEEEEEEEEEEEEEEEEEEEEE]------[EEEEEEEEEEEEEE]----------------------
----[EEEEEEEEEEEEEEEEEEEEEEE]------[EEEEEEEEEEEEEE]----------------------
------[PPPPPPPPPPPPPPPPPP]------[PPPPPPPPPPPPPPPPPP]---[GGGGG]----------
------[PPPPPPPPPPPPPPPPPP]------[PPPPPPPPPPPPPPPPPP]---[GGGGG]----------
------[PPPPPPPPPPPPPPPPPP]------[PPPPPPPPPPPPPPPPPP]---[GGGGG]----------
------[PPPPPPPPPPPPPPPPPP]------[PPPPPPPPPPPPPPPPPP]--------------------
---------------------------------------------------------------------
---------------------------------------------------------------------
---------------------------------------------------------------------
Key:
[E]  - Enemy Unit
[P] - Your PG Unit
[G] - Your Great Eagle
As  you can see in this scenario, your two blocks of PG have just charged  your opponents two enemy units. Your GE flies up to the right. You also  notice the size and threat of the two enemy units that you just charged.  You look at the larger enemy unit to the left. You realize that you  have a good chance of tying/holding against that large unit with your  PG, but you do not think you can beat it alone. You also look at your  opponents smaller unit on the right. You think you can definitely beat  that unit alone with your PG unit, but it may take two rounds of combat.  Because you were smart and looked ahead, you moved your eagle to the  right of the combats, and the reason why will become clear soon.
So  the battle goes on. You turn out to be right, neither units go anywhere  on your combat phase. Then it goes over to your opponents combat phase.  Because you have numbers on the smaller enemy unit now, you break it  and it flees. However, you decide not to chase after it with your PG  unit. You instead reform looking to get a flank charge against your  opponents larger unit, and you let the smaller unit get away, because  you know that you need the help in the other combat.
So it is the  beginning of you're turn now, and the scene looks like this:
--------------------------------[EEEEEEEEEEEEEE]----------------------
--------------------------------[EEEEEEEEEEEEEE]----------------------
---------------------------------------------------------------------
---------------------------------------------------------------------
----[EEEEEEEEEEEEEEEEEEEEEEE]------------------------------------------
----[EEEEEEEEEEEEEEEEEEEEEEE]------------------------------------------
----[EEEEEEEEEEEEEEEEEEEEEEE]-------------[PPPPPPPPPPPP]-----------------
----[EEEEEEEEEEEEEEEEEEEEEEE]-----------[PPPPPPPPPPPP]-------------------
----[EEEEEEEEEEEEEEEEEEEEEEE]--------[PPPPPPPPPPPP]----------------------
------[PPPPPPPPPPPPPPPPPP]--------[PPPPPPPPPPPP]-------[GGGGG]----------
------[PPPPPPPPPPPPPPPPPP]------[PPPPPPPPPPPP]---------[GGGGG]----------
------[PPPPPPPPPPPPPPPPPP]----[PPPPPPPPPPP]------------[GGGGG]----------
------[PPPPPPPPPPPPPPPPPP]---------------------------------------------
---------------------------------------------------------------------
---------------------------------------------------------------------
---------------------------------------------------------------------
So  what do you do? You declare two charges. The first is your PG unit into  the flank of the large enemy unit. The second, however, is your GE into  their fleeing small enemy unit. You know that if that enemy unit  rallies, you will have then gotten no points for it because of the new  rules, so you chase it down with your eagle. You charge with your eagle  and get the average of 19" charge range. Because they were only  approximately 7" away from your eagle, there is close to no way they can  flee far enough (and they have to flee, as they were already doing so)  from your eagle, so once again that little 50 point model has made up  for its points three fold or more, and allowed you to turn the tables on  the other larger enemy unit.
Trick #3.2, 'Chasing Away', special thanks to Ptolemy
Here  is another similar thing you can do, that helps you clear those smaller  units on the board and gain you a few extra points in the game. When  you charge a unit, and you know that they are going to flee, then you  can charge them once again with an eagle. That way, they are forced to  flee once again, either to be caught by your eagle, or run off the  board. If neither of those things happen, then it is well worth moving  that unit another 2d6" away from the action. Remember, your eagle only  has to be within 22" of that unit to be able to declare the charge.
Trick #4, 'The Speed Bump'
This  is a simple trick, but I find that it works rather well in slowing your  opponents down to give you more time to shoot/magic their big combat  units to give you a combat edge when they get to you. Pretty much all  you are doing is flying your eagle up to their big units, then angling  the eagle so that if the eagle is charged next turn, the big combat unit  will overrun in a direction they do not want to go, or they can not  charge and reform to get out of the way of your eagle, which you will  then just move back and do it again, or they can charge, kill your  eagle, and then not overrun, and move a total of one inch in their turn,  instead of their typical 8-10" advancement on your line.
Trick  #4.2, 'The Double Block', special thanks to geoguswrek!
This  is very similar to 'The Speed Bump', however you would use it when it  is not guaranteed that you're eagle would hold, ie. your opponent causes  Terror. In this trick, you put two eagles blocking your opponents unit,  and when he declares a charge against the first, you flee, but then he  can only redirect into your other eagle. You flee a second time with the  other eagle, and because each unit can only redirect one time in each  turn (see page 18), your opponent is then forced to go after the second  eagle. If you set it up right, this can lead your opponent into a bad  position, sometimes setting you up for a flank charge.
Trick #5, 'The Place Holder'
This  may just be my favorite trick of all to use in a game, because it  catches your opponent off guard, and can help you get out of very tricky  situations. Some of you may already know this trick from reading my  past BatReps, and if so, kudos to you for reading my BatReps! Anyways,  here is a situation where this trick could be applied:
---------------------------------------------------------------------
---------------------------------------------------------------------
---------------------------------[EEEEEEEEEEE]------------------------
----------------------[EEEEEEE]---[EEEEEEEEEEE]------------------------
---------------------[EEEEEEE]--------------------------[GGG]--------
--------------------[EEEEEEE]--------------------------[GGG]--------
-------------------[EEEEEEE]-----------------------------------------
------------------[EEEEEEE]-------[EEEEEEEEEEE]------------------------
---------------------------------[EEEEEEEEEEE]-----------------------
--------------------------------------------------------------------
------------------------------[PPPPPPPPPPPPPPPPPP]--------------------
------------------------------[PPPPPPPPPPPPPPPPPP]--------------------
------------------------------[PPPPPPPPPPPPPPPPPP]--------------------
---------------------------------------------------------------------
---------------------------------------------------------------------
---------------------------------------------------------------------
As  you can see, your opponent has put a weak unit right in front of your  PG unit, in hopes that you will charge it, beat it, and run it down. If  you did this, then you would run into his second smaller unit. However,  in doing so, you will have fallen into his trap, as he will then be able  to flank you with his larger, more formidable unit. So what do you do?  You declare two charges. Your eagle is conveniently placed on your right  flank, so you charge the small unit right in front of your PG with your  PG, then you charge the small unit behind that with the eagle. In doing  so, you then proceed to beat that unit and chase it down with your PG,  running into the small unit behind it. However, because you flanked that  unit with your Eagle, you can then fight another round of combat in the  same combat! You then can beat and run down that second unit with your  PG, and in doing so get out of harms way from the flank charge.
Trick #6, 'Placement Stalling', special  thanks to wamphyri101!
I believe that the placement of  units at the beginning of the game is the one largest thing that will  make or break the result of any game. Great Eagles, believe it or not,  can give you quite the edge with this, and here is how. The very first  thing you place down at the beginning of the game is your eagles. The  more eagles you have, the more units your opponent will have to place  down for you to see before you start putting down actual units. This  way, you can 'pick your fights', so to speak, and place your units the  way you want them in relation to your opponents units. I typically try  to put my GEs on the flanks to begin with, because with their 20"  movement they can get were they are needed relatively easily.
Trick  #7, 'Combat Res Generator', special thanks to Xarhain!
Xarhain wrote:
One  more basic tactic is the combat res generator. Often you'll have flown  behind enemy lines to attack a war machine or attempt any number of the  tricks listed above, but for some reason the eagle is no longer  required. Maybe the cannon blew itself up or the unit you were stalling  got purple sunned. Either way, you now have a free eagle you can charge  into the back of a stalling combat. With the new lack of unit strength  you get +1CR for charging and +2CR for the rear charge. You'll do a  wound against standard infantry and maybe take one back. Instant +3CR,  and a speedy unit involved in the combat to chase them down once they  break!
Overview:
I  guess what I'm really trying to tell everybody is this: 
think  ahead! If you can think ahead a couple of turns, you can  set up these neat tricks, and avoid tricks that your opponent tries to  set up against you. A lot of this comes from me being an avid chess  player, because in chess it is all about thinking ahead. I use the same  skills in warhammer, and so far I am doing pretty well. 
 
Commentary is welcome and  appreciated.
Since some of the diagrams might come out funny on this site, here is a link to the original:  http://www.ulthuan.net/forum/viewtopic.php?f=66&t=33028&start=0